We will use quantitative techniques to measure the protective responses of normal skin to UVR as a function of heritable melanogenic capability and as a function of ultraviolet waveband. We will examine both melanin and non-melanin mechanisms. For the first time we will correlate structural and optical measurements in vitro with protection against erythema and against specific markers of epidermal and dermal damage. Responses will be compared in individuals of different genetic tanning capability and in response to different wavebands (UVA and UVB). UV-induced structural changes and alterations in optical transmissions by isolated epidermis and stratum corneum samples will be measured and correlated with the development of tolerance to insult by UVR. Tolerance will be assessed by measuring the degree of erythema, structural damage to dermal blood vessels, and damage to DNA in the epidermis resulting from further UV exposure. These studies will determine the relative importance of different protective mechanisms in subjects of different heritable pigmentation capabilities. We will determine whether Caucasians are heterogeneous with respect to their ability to develop tolerance to UVR by non-melanin mechanisms and, if so, whether differences correlate with or are dependent of tanning ability. The effects of UVA exposure upon melanocyte populations will be examined in different skin types, and the functional contribution of longwave induced changes to induced photoprotection will be examined.